Aircraft landing gear



April 1947- J. B. WASSALL ET AL 2,418,325

AIRCRAFT LANDING GEAR Original Filed May 21, 1940 5 Sheets-Sheet l INVENTORS. JOHN B.WAssAu AND BY film-132M WLLACE- ATTORNEY April 1, 1947. J. B. WASSALL ET AL I 2,418,325

AIRCRAFT LANDING GEAR Original Filed May ,21, 1940 3 Sheets-Sheet 2 OHN B WASSALL AND April 1, 1947. J. B. WASSALLET AL 2,418,325

AIRCRAFT LANDING GEAR Original Filed May 21, 1940 D 3 Sheets-Sheet 3 FIG. 4. 48

INVENTORS. JOHN B. WASSALL AND Y ILFRED N. WALLACE.

Patented Apr. '1, 1947 AIRCRAFT LANDING GEAR John B. Wassall, Burbank, and Wilfred N. Wallace, Glendale, Calif., assignors, by mesne assignments, to Lockheed Aircraft Corporation, a corporation of California Original application May 21, 1940, Serial No.

336,414. Divided and this application Decemher 6, 1941, Serial No. 421,950 7 6 Claims. 1

This invention relates to retractable and steerable landing gears for aircraft and more particularly to mechanism by which the steering movements may be effected.

The present application is a division of our co-pending application Serial No. 336,414, filed May 21, 1940, which issued September 7, 1943, as Patent No. 2,329,168.

In aircraft it has heretofore been common practice to accomplish steering of the vehicle on the ground by the use of a steerable tail skid or tail wheel. At the present time, however, there is a growing trend to revive the well known tricycle arrangement of the landing wheels wherein a single wheel is disposed Well forwardly under the craft and a pair of laterally. spaced wheels are disposed aft of the center of gravity. Such an arrangement also has the advantage of keeping the aircraft in the same horizontal attitude on the ground as is maintained in flight.

2 a source of fluid pressure to opposite ends of a steering motor under the complete control of an operator.

A further object is to provide such a valve which is of relatively simple and sturdy construc tion but which at the same time will yield a positive control while being sensitive to the control forces to a high degree. Another object is to provide in a steerable landing gear unit, means to arrest torsional vibrations, generally known as landing gear shimmy. which may be set up in the landing gear as the result of passage over rough or uneven ground surfaces and other causes.

Still another object is to provide a steering.

This general practice was Widely adopted in the early days of flying but then was entirely neglected through all of the subsequent period of rapid development of the airplane. Due to its inherent directional stability, desirable at high landing speeds and under conditions of uneven wheel braking, it is particularly desirable of use in modern aircraft of all sizes. However, very few satisfactory arrangements of retractable tricycle landing gears, particularly retractable nose wheels, have been either proposed or built as a result of difilculties encountered in arresting shimmying, providing suitable steering mechanism, stowage space, points of attachment and other conditions.

Accordingly, it is an important object of this invention to provide mechanism adapted for use with the type of landing gear installation described above by which it may be rendered steerable so that aircraft movements can be controlled when the craft is moved about on the ground. Although the present steering mechanism and arrangement are designed for use with a landing gear installation of the particular type described, it is by no means limited to such use and the following illustrative description of the steering system in connection with a particular gear arrangement is not intended as a limitation of its application to other gear arrangements.

Another object of this invention is to provide certain novel components for incorporation in.

this steering system, including a steering control or hydraulic switching valve adapted to connect automatically damping torsional vibrations occurring in the landing gear system and of im-' parting steering movements to the ground engaging element of the system. V

A complete understanding of the invention m'ayi be had by referringto the annexed drawings in connection with the following specification.

In the drawings:

Fig. 1 is a side phantom View of part of an aircraft showing a landing gear and the operating elements associated therewith;

Fig. 2 is a front elevation of the same landing gear;

Fig. 3 is a plan view of the bracing framework for the landing gear;

Fig. 4 is a bottom plan view of the drag or breaking strut elements; 7

Fig. 5 is an enlarged side view of the joint. portion of the strut shown in Fig. 4;

Fig. 6 is a cross-sectional view of a valve forming part of the hydraulic steering system operating on the landing gear;

Fig. 7 is a diagrammatic showing of the hydraulic steering system; and

Fig. 8 is a cross sectional view of the valve means I62 forming a part of the system shown in Fig. 7.

Referring first to Fig. 1, there is shown in phantom outline a portion [0 of an aircraft and in this portion is arranged the component parts which make up this invention. As is usual the landing gear terminates in some form of ground engager which in the present instance is shown to be a rubber tired pneumatic wheel l2. This wheel is carried by a stub axle [4 which has extensions l6 and I8 projecting on either side of the wheel. The purpose of these extensions will appear later.

The wheel axle I4 is rigidly carried by a curved bracket 20, which at its upper end rigidly connects to a telescopic shock strut generally designated 22 and consisting of a lower piston element 24 telescopically received in an upper cylinder element 26. The assembly so far described is embodies an oleo unit within the telescopic halves 24 and 26. This assembly being well known in the art, it is not considered necessary to describe it in further detail.

However, the mounting of the shock strut 22 is a part of this invention and comprisesa framework 28 (see especially Figs. 2 and'3) which rigidly supports the wheel-carrying strut but yet allows for-turning movements of the strut 22 about its OWn axis. The framework consists of a-centrally located collar 3% into which the upper partyZGo-f the shock strut 22 fits. This collar is braced and welded to a straight tubular member 32 which terminates-at each end in trunnion stubs 34-, which are journalled within suitable supports provided uponthe engine or the aircraft structure, and which mount the framework 28 so that it can be rotated with respect to the aircraft body portion [0.

Extending from the collar 38 at right angles to themember 32 and in the same plane therewith is a short tubular member 35 which intersects with a bow shaped tubular member 38 at the latters midpoint (see Fig. 3). This member 38 curves rearwardly from its middle to rigid connections with-the ends of member 32. Also, as shown-in Figs. 1 and 2, a pair of rigid braces 40 each slant downwardly from these same ends of member 32 to a bolted connection at 42 between ears of the collar 44 which surrounds the lower end of oleo cylinder 26. There is a sliding fit betweenme collar M and the strut is journalled in thiscdllar in the same manner as in collar 30 in the frame 28. Such provision for turning movements of the strut adapts it to be steered under the control of mechanism to be described nter: Also'to be described hereinafter is mechanism to cause retractive swinging of the landing g'ear'including the framework 28 about the trunnion stubs 34.

the extended position of the landing gear, it is braced in the fore and aft direction by means of a breaking Y-shaped strut '45 shown in more detail in Figs. 4 and 5. This strut is fitted at each 'of' its three extremities with bearing eyes 48. At the front end the eye 48 is inserted b'etween-apertured ears extending from the collar 44. A bolt extending through the ears and the eye holds the two together and eifects a pivotal connection between the strut 4t and the shock strut 22. The strut 46 consists of a short fore portion 52 and a considerably longer rear portion 54 which fork into a pair of symmetrical arms 56. The portions 52 and 54 are joined together by means of a pivot joint 58 which preferablyis'ofiset slightly below the center line of the'strut ifias shown in Fig. 5. Abutments Bil are provided on both sides of the joint line which contact each other when the members 52 and 54 lie inth'e same plane. This allows the joint to be broken in only one direction and makes the brace rigid in all other respects.

Another feature of the present joint is the rigid member '62 which projects upwardly from strut portion 54 and then inclines forwardly over the meta more or less conventional in aircraft use and 4 joint line for an appreciable distance. This member is fitted with a bushing 64 adapted to receive a spindle carrying a pair of grooved discs or heaves 66. An identical bushing 63 is provided on the strut portion 52 which receives a spindle carrying a second pair of identical sheaves 1E) spaced on either side of the bushing. Strong rubber loops or shock cords 12 are stretched over each pair of discs 5'5'IEI to thus bridge the joint of the breaking strut 45. At this joint 58, on either side thereof are identical pairs of spaced plates it which extend from cylindrical bushings 5-6 on either side of the joint. When the joint is broken to retract the landing gear to the position shown by dotted lines in Fig. 1, the rubber rings or loops i2 are strletched, since the spacing between the discs 66 and it, which normally is only enough to slightly tension the rings, becomes reater. lates it moves with the strut portion 54 to straddle one of the rubber rings 12 which latter is thereafter contacted by the bushings l6 and further stretched thereby to a maximum tension at the fully retracted position of the landing gear. This organization provides a resilient termination to the retracting movement. On the other hand it also provides an initial force to aid in starting the unfolding of the mechanism of the landing gear for movement to the fully extended or aligned position. The sheaves and loops have been omitted from the strut details in Figs. 4 and 5 for the sake of clarity.

Another purpose served by the structural member 62 is the support of a micro-switch housing 78 (see Fig. 5i connected by an electrical cable 48 which, as shown at 88 in Fig. 1, serve for the attachment of the strut to structural members 90 of the framework of body iii. The arms 56 are also fitted with aligned and spaced bushings 92 adapted to receive a spindle which passes through an aperture in the end of a piston rod 94. The

end of the rod is connected between the bushings 9-2 with the connection being such that the rod can pivot on the spindle. A hydraulic cylinder 96 is provided into which the rod 94 extends where it terminates in a suitable piston head located in a cylindrical chamber provided at either end with flexible fluid conduits 98 and Hill. These conduits extend along an extension I02 of the cylinder 96 to connection with a reversing valve (not shown) but of any suitable type well known to the art by which either end of the cylinder 96 may be" corinected to a source of fluid pressure for thepurpose of moving the rod Q4 telescopically with respect to the cylinder. Because the extension I02 is' pivotally mounted at lilon the structural members 96, the drawing of the strut 94 into the cylinder will first break the joint 58 and if the action is then continued there will follow a con-. current pivoting of the cylinder 96 about the point IE4; of the strut portion 54 about the points 88; and of the framework 28 'on the trun-v nion stubs 34 from the full'line or extended 1 0- sition to the dotted line or retracted position. As before explained the framework 28, when it At the same time each pair of spaced pivots, carries with it the landing gear strut 22 and the ground engager I2 to the retracted position where all of the gear except for a part of the wheel I2 are completely housed within the carry an emergency load when it is in such an unnatural position the fork arms are provided with adjustable abutments I06 and I08. Then when the load occurs on the wheel the axle extensions I6 and I8 are forced to contactthe faces of these abutments (if the contact has notpreviously occurred) whereupon the loading on the wheel Will be distributed through the fork arms.

56 and thus into the framework 90. It may be noted at this time that due to the forked construction of the strut portion 54, the wheel I2 passes between the fork arms 56 and it is therefore possible for the axle to make contact with or bridge the abutments I06 and I06.

The remaining feature of this invention, yet to be described, is perhaps the most important part thereof, accomplishing as it does the dual functions of automatically attenuating any torsional vibration or shimmy which may be set up in the landing wheel-shock strut combination and of providing a means by which the Wheel may be steered during ground travel.

Before entering the description of this mechanism it should be noted that the two telescoping parts 24 and 26 of the shock strut would otherwise be free to rotate With respect to each other but in accordance with present practice a nut-cracker linkage consisting of arms H0 and I I2, pivotally joined at H4, and respectively pivotally connected at H6 and H8 to collars I and I22 on the shock strut portions 24 and 26 is provided for the purpose of preventing such relative turning. This linkage also serves to transmit steering movements applied to the top of the strut 22 to the wheel I2; the collars I26 and I22 being fixed to their respective strut portions.

Such steering movements are limited to a prescribed angle by spaced stops I24, I26 on the tubular member 38. (see Fig. 2). Operating in the space between these stops is a contact element I28 fixed to a projection of a collar I30, which is in a fixed surrounding engagement with the strut portion 26 and therefore partakes of any of its movements.

w Steering movements are applied to the shock struct through a connection to the same projection I30 of the collar 30. This connection is organized for pivotal movement at the point I32 and comprises a piston rod I34 terminating in a piston I36. Enclosing the piston is a cylindrical fluid tight casing I36 which is pivotally attached at I40 to one end of the framework 28. This pivotal mounting of the cylinder allows it to align itself at all times with the piston rod I34 following steering movements of the shock strut. The piston-cylinder combination I32--I38 constitutes an hydraulic unit controlled from a remote point in the aircraft and is the immediate means by which steering movements are imparted to the strut 22 since fluid pressure admitted to one side or the other of the piston will cause movement thereof.

- Pressure lines I42 and I44 lead from the ends of cylinder I30 to a distributing control valve I46. This valve is preferably mounted on the breaking strut portion 54 bystraddling the fork arms 56 and being bolted thereto as shown in Fig, 1. Controlof the valve is accomplished from a remote point by means of cables I48 and IE0 which pass over sheaves I52 and I54 by a suitable pedal, or other operating lever, not shown but of a type commonly employed for such control purposes. The valve I46 is also connected to a source of pressure P by a direct flexible fluid conduit I56 and a return line I58 leading to a liquid reservoir R (see Fig. 7)..

7 Referring now to Fig. 6 for details of the control valve I46, it is seen to comprise a fluid-tight casing I60 of a generally cylindrical shape. The fluid conduits I42 and I44 extend from either end of the valve casing to their connection with the servo unit cylinder I38. Each of these conduits includes a check valve indicated at I62 in Fig. 2 and shown in some detail in Fig. '7. These check valves are of well known construction being of the type which are normally held open to permit of normal or steady passage of fluid. Springs are employed to keep the valves open but they are so organized that any sudden rush of fluid flowing outwardly from the unit I38 toward the, valve I46 has the effect of instantaneously closing the valve. This characteristic is taken advantage of to attenuate shimmy effects introduced to the landing wheel and its shock strut by passage over the ground. When such vibration occurs in the system the piston is rapidly oscillated and this sets up pulsations in the contained liquid with the result that the check valves immediately close and the relatively incompressible liquid employed in the cylinder I38 is trapped on both sides of the piston I36. When so trapped it acts to absorb the vibration effects and prevents them from being transmitted into the aircraft structure.

In detail, check valve I62 is interiorly formed, as best shown in Fig. 8, with a hollow cylindrical chamber substantially consisting of two portions 2I0-2I2, with the first being of larger diameter than the second. The movable valve element or disc is designated by the numeral I98 and is carried by a rod 200 which is reciprocably mounted on the centerline of the chamber, being supported at one end in a spider bearing 204 and at the other in a similar bearing support 206. A lightweight spiral spring 202 is provided on the rod 200 to react between the spider 204 and the valve disc I96. The effect of the spring is to normally maintainthe valve disc at its extreme lefthand position out of contact with the face 2I4 of the chamber wall which forms a valve seat for the disc. When the disc is forced against this face, as explained previously, flow through the valve into the conduit I44 is prevented. In order to prevent the valve disc from bein forced by the spring into contact with the face of the spider 206 which would thereby prevent fluid flow through the apertures 2I6 formed in the spider, the latter is formed with a boss 208 which serves as a spacing stop for the disc. Thus with the disc I68 held thereagainst, uninterrupted flow is permitted through the valve in either direction as long as the flow characteristics remain such as not to effect closing of the valve.

- The system for producing steering movements will now be considered, starting with a detailed description of the inner mechanism of the distributing valve I46. As before stated, the casing I60 is of a generally cylindricalform and is hol-' low' butit QISOiSJfOImGd with an ofiset'lporti'on of triangular shape. This portion at the apex of the triangle receives the end of the exhaust conduit I58 as shown at I64. Atthis point within the casing is'formed a rotatable three-Way valve I166 operated by a double armed lever I68. vAtthe ends of the lever are pivotally connected'the steering cables I48 and I56 and consequently when the rudder pedals, to which these cables are preferably connected, are moved in one direction or the other, it follows that the lever I68 is moved in exactly the same manner. The valve I66 will concurrently be rotated in one direction orthe other away from the neutral position shown. EX- tending'from the valve is an arm III- fitted at its end with a ball or sphere I12 located exactly in the center of the length of the cylindrical portion of the casing I66. Adjacent this sphere in the side-of the casing at 114 is the connection of the conduit I56,

Also within the cylindrical part of the casing and centered therein is a cylindrical spring housing I-'I.6 which at its midpoint has a well "'8 into which the ball end I12 of the lever I fits. Accordingly, when the ball I72 moves to one side or the other in response to operation of the cables I48 and I50, it presses on the spring housing I16 and causes it to be displaced along the line of its longitudinal axis. "The central well divides the spring housing into two cylindrical chambers each containing a helical spring I60 surrounding a valve stem or rod I62 which extends outwardly through an aperture inthe end wall of the housing I16.

The end of each rod I62 inside the housing carries a member I34, attached thereto and slidable within the cylinder. The spring I80 is confined between the head 184 and the end Wall of housing H6 and is normally compressed. Under this arrangement the springs tend to draw their associated rods I82 into the housing but since the rods carry circular valvediscs I 66 on their opposite ends which seat against valve seats I88 formed in the inner wall of the casing I60, the rods can only be drawn in until the valve discs become seated. With these discs I86 seated, the fluid pressure entering the casing I60 through the conduit I56 is confined to the space between the discs I86.

At each end of casing I 60 are chambers I90 and I92 set off from the remainder of the cylindrical portion of the :casing'bythe valve discs I86. These chambers are respectively in communication with the opposite ends of the servo unit I38 by means of the conduits I42 and I44. They are also in communication with the ports in the valve I66 through passages I94 and I66 formed through the triangular portion of the valve casing I60. With the valve I66 in central Or neutral position, both of the passages I94 and I96 are in communication with the exhaust conduit I58 which leads to a suitable fluid reservoir. But if the valve I66 is turned in either direction, one or the other of said passages is cut off from such communication due to the shape of the ports in the valve.

The operation of valve I46 is as follows: Pushing the right rudder pedal pulls cable I50 and this gpull, transmitted through lever I68 to lever I70, causes spring housing I16 to be moved'to the left, reducing the pressure on the left hand spring I80, thereby allowing'the left hand valve disc I86 to open under the influence of the hydraulic pressure exerted within-the inner chamber open to the fluid source through inlet I56, At the 8 same time ithec passage I94 is cut off from com munication with the return conduit I 58 .bycountor-clockwise movement of valve I66. The liquid in conduit I56, deliveredsfrom a hydraulic pump or other pressure source, then flows to the left end of the servo unit or steering cylinder 138 through conduit I62. This causes theipiston I36 to move to the right, and turn the strut 22in the samedire'ction whereby the wheel I2 would cause the'aircraft to turn in the same direction. The movement of piston I36 forces the contained liquid out of the right end of cylinder I 38, through conduit I44, chamber I62, passageway J I96, valve I66, and-conduit I58 to the reservoir.

Ground reaction on the wheel [2 willr'nake the restoring moment proportional to the displacement. Therefore a constant pressure will holda constant displacement. Similarly, pushing the left rudder pedal, instead of the right, will produce a directly opposite steering efie'ct and this will be so even though one or the other of the valves I62 may be closed for the hydraulic pressure will be sufiicient to open the valve with the assistance of the valve spring.

Due to the particular internal arrangement of the valve I46, the conduits M2 and I44 are normally both in open communication with the return line I58 leading to the reservoir. This 'condltion will remain fixed so long as the valve I46 remains in the neutral or unoperated state. At the same time any excess pressure developed by the hydraulic pump P, while the valve is in neutral, will be relieved by overcoming the resistance of the similar springs I60 and forcing the valve discs I86 away from their seats. When this occurs, a relieving flow of pressure fluid can take place into the chambers I90 and I92 and this excess of fluid can flow on into the return line I56, through the passageways I94 and I96. Since these two passageways are in open communication, no unbalanced pressure can build up in either end of the cylinder of the hydraulic motor I38. Thus the pump P can be continuously operated, with fluid circulation being diverted at the valve I46 back to the reservoir R. However, as previously explained, as soon as the valve is operated, this circulation path is interrupted and the balanced pressure conditions in the conduits I42 and I44 no longer hold, with the result that unbalanced pressure is applied to (:31; side or the other of the operating cylinder While this invention has been described and illustrated as being embodied in anose wheel installation suitable for use in a tricycle landing gear. it will be perfectly obvious to those skilled in the art, after understanding the invention, that the same characteristics, or as many as are desirable, could be applied to the other Wheels of a tricycle landing gear or for that matter to any landing gear. It is also obvious that various other changes or modifications may be made therein without departing from the spirit or scope thereof. It is aimed inthe appended claims to cover all such modifications and .changes.

What is claimed is:

1. A selective control valve of the balanced type comprising a casing, a transverse cylindrical chamber disposed within said casing, chambers disposed at the opposed ends of said transverse chamber, said casing housing a rotary'valve body centrally disposed substantially equidistantly from said end chambers, a ported valve element rotatably mounted within said valve body, afluid pressure inlet in communication with said transverse chamber, conduits inlcommunication with each 'of said end chambers, a fluid passageway connecting each of said end chambers withsaid rotary valve, a return line in communication with the saidv rotary valve, valve seats disposed between said transverse chamber and each said end chamber, a valve housing reciprocable transversely within said transverse chamber, valve discs normally in closedrelationship with each said valve seat resiliently carried through the opposed ends of said valve housing by means of compression springs carried therewithin, said rotary valve having a ball ended lever portion engaging the central portion of said valve housing and means to rotate said ported rotary valve whereby said valve housing is concurrently moved to permit fluid pressure to pass through said leading valve into said adjacent end chamber and simultaneously prevent fluid passage from said end chamber to said return line.

2. A hydraulic system for the actuation of a movable member subjected to high frequency oscillatory movements, comprising a control valve, a source of fluid pressure connected to said valve, a pressure cylinder mounted adjacent to said movable member, conduits linking said valve to said cylinder, piston means reciprocable within said cylinder operatively attached to said member to impart movements thereto depending upon control movements of said valve from its neutral position, resilient means acting to restore said valve to said neutral position, the said valve having fluid passageways open in said neutral position of the valve to provide fluid connections between said conduits, and further valve units connected in series with said conduits and operative to prevent the fluid transmission of said high frequency oscillatory movements of said member through the hydraulic system from said pressure cylinder to the said control valve.

3. The combination with a hydraulic power plant for the damping and steering of an aircraft nose wheel including a fluid pressure source, a reversible fluid motor, a selective control valve of the balanced type interposed within fluid conduits operatively interconnecting said fluid motor with said pressure source, a fluid reservoir and a return conduit from said control valve thereto, and normally open passageways operatively associated with said control valve interconnecting the ends of said fluid motor to the return conduit to said fluid reservoir, of a normally closed automatic valve means operable to bleed excess pressure developed by said pressure source through said control valve equally into said passageways, said valve means including a reciprocable valvercarrying member, valve units operatively and resiliently carried at the opposed terminals of said valve carrying member whereby movement of said valve carrying member in one direction permits opening of its leading valve and continued closing of its trailing valve, exhaust valve means operatively connected to said valve-carrying member interposed at the junction of said fluid passageways with the return conduits to said reservoir for the selective closing of either said passageways, and manual control means for said valve means selectively operable to open one unit to place one of said passageways in communication with the said pressure source while the said exhaust valve means operates to block the communication of the same passageway to the return conduit to said reservoir.

4. A selector control valve of the balanced type comprising a casing, an elongated cylindrical 10 chamber within said casing, valve seats oppo-' sitely disposed within said cylindrical chamber adjacent the ends thereof, a reciprocable valve housing disposed within said cylindrical. chamber, valve elements engageable with said valve seats resiliently carried through the opposed ends of said reciprocable valve housing by resilient means carried therewithin, a, ported rotary valve in fluid communication with said valve chamber, a fluid pressure inlet in communication with said cylindrical chamber, fluid conduits in communi: cation with each end of said cylindrical chamber and Withseparate ports of said rotary valve, a, return line in communication with said rotary valve, said rotary valve having an operative engagement with said reciprocable valve housing and manual means for the rotation of said rotary valve whereby said reciprocable valve housing is concurrently moved, to permit fluid pressure to pass through its said leading valve into the adjacent end of said cylindrical chamber and to simultaneously prevent fluid passage from the same end of said cylindrical chamber to said return line.

5. The combination with a hydraulic power plant, including a closed fluid circuit interconmeeting a pressure source and a reversible hydraulic motor, a control valve bridging said circuit intermediate the motor and pressure source, normally open passageways within said valve connecting both sides of said motor to the circuit return to said source, and a rotary valve operatively associated with said control valve adapted to selectively close the said passageway on that side of the circuit to which the said control valve may be opened, of a normally closed automatic valve assembly operable to bleed excess pressure developed by said source equally into said passageways, said valve assembly comprising a reciprocable valve-carrying member, valve units oppositely and resiliently carried at the opposed terminals of said carrying member whereby movement of said carrying member permits opening of its leading valve and continued closing of its trailing valve, and control means for said valve assembly and said rotary valve selectively operable to open one of said valve units to place one of said passageways in communication with the output of said pressure source while the said rotary valve blocks the communication of the same passageway to the circuit return to said source.

6. A hydraulic system for the actuation of a movable member subject to both controlled movements and uncontrolled oscillatory movements, comprising a fluid motor operatively connected to said movable member, a control valve having a pair of outlet conduits in fluid communication with the ends of said fluid motor, a source of fluid pressure, a fluid conduit from said pressure source to the inlet of said control valve, a bypass passageway in said control valve normally open in the neutral position of said control valve providing fluid communication across the said valve outlet conduits communicating with the ends of said fluid motor, a return line for fluid from said bypass passageway, said control valve arranged in its operative positions: to selectively provide fluid pressure from said. source to either end of said fluid motor for the controlled actuation of said movable member, and further valve means disposed within said valve outlet conduits between said control valve and the ends of said fluid motor operative in the neutral position of said control valve to prevent 11 higher rates of fluid flow initiated byhigh frequency oscillatory movements of .said .movable member'during its *said uncontrolled, condition.

JOHN vB. WASSALL. 'WILFRED N. WALLACE.

REFERENCES CITED 7 The-followingreferences are of record in the file'of this patent:

UNITED STATES PATENTS Number Name Date 394,314 Turner Dec. :11, 1888 2,220,920 Stratton Nov. 12, 1940 1,425,086 Geer Aug. 8, 1922 2,202,960 Parker June 4, 1940 Johnson- June 26, 1906 Number 

